Instrument for strain testing



Oct. 23, 1956 R. s. sTRlMEL INSTRUMENT FOR STRAIN TESTING Filed Aug. 29,1955 BY @wf/M ATTORNEYS United States Patent() N' INSTRUMENT FORSTRAINIESTNG Robert S. Strimel, Penllyn, Pa., assignor to Tinius OlsenTesting Machine Company, Willow Grove, Pa., a corporation ofPennsylvania Application August 29, 1955, Serial No. 531,121

3 Claims. (Cl. 33-148) This invention relates to materials testingequipment and, in particular, relates to instrumentation for measuringthe elongation or strain lof a specimen to be tested in tension. i

In the tension testing of -a specimen of metal or plastic or the Ilike,the specimen is secured in grips between the crossheads of a testingmachine and strained by causing one crosshead to move relative to theother. The present invention is concerned with an instrument to beclamped on the specimen and operative to automatically measure thestrain and to provide an electrical coil signal for apparatus to rec-ordthe same, for example, a recorder `as disclosed in my copendingapplication, Serial No. 261,239., led December l2, 1951, and entitledRecording Device for Use in Ccnjunction with Testing machines'. Aninstrument of the kind in question is commonly called an extensometer inthe art.

in general, the invention contemplates an extensometer comprising a mainbody or frame carrying a differential transformer, a fixed and a movableknife edge, the movable edge being arranged to move the core of thetransformer together with a spring-loaded roller adapted to engage aspecimen to be tested and hold or clamp the same against the knifeedges.

The-re are several important features of the invention, and among theseare: y

A mounting for the roller providing for the same to be moved toward andaway from the knife edges so as to accommodate specimens of differentsizes, the arrangement including a spring adapted to push the rolleragainst a specimen, the :spring being arranged so that regardless of theadjusted position of the roller, the clamping force remains constant.

Another feature of the invention is that the interconnection between themovable knife edge and the transformer core includes a pivot whichenables the movements of the knife edge and the core to be substantiallylinear with respect to one another.

Another feature of the invention is that the pivotal interconnectionbetween the transformer core and the movable knife edge provides for thecore to very accurate-ly follow the motion of the knife edge.

Another feature of the invention is that the various components thereofare so constructed and arranged that the -device can be -made in anextremely small size.

The manner in which the foregoing features are accomplished and otherfeatures of the invention will be apparent from the followingdescription and drawings wherein:

Figure l is a side elevational view of the device as connected to a testspeci-men;

Figure 2 is an end view looking toward the left in Figure l;

Figure 3 is a plan view of Figure l;

Figure 4 is a detailed view of the frame or body of the device;

Figure 5 is a section taken on the line 5-5 'of Figure 3 with certainparts omitted; and

2,7624@ Patented Oct. 23, i956 Figure 6 is a plan section taken o-n theline 6-6 of Figure 1 showing the device in unclamped position.

In general, the device includes two knife edges k-l and k-Z which areadapted to engage a test specimen S and be held firmly against thespecimen by a pressure roller R. The upper or movable knife edge k-l isinterconnected to a transformer T, which is also part of theextensometer. As a specimen is stressed in tension, the upper knife edgek-l follows the elongation and this motion is imparted to thetransformer to produce a signal usable in the recording yapparatus of myabove-mentioned application.

With reference to Figures 1 and 4, it will be seen that theexten'someter has a main body or frame 1 comprising a downwardlyextending portion 2 together with two upper portions 3 and 4 which are'arranged to form an elongated channel 5. Disposed within the channel 5is a llever arm 6 which carries the knife edge k-l. Within the lever aum6 is an insert 10 (see Figure 2), the ends of which are conically-shapedas indicated by the dotted lines 11 and l2.. The upper portions 3 and 4respectively carry screws i3 and 14, which may be adjusted in the upperportions and locked in place by the nuts 15 and 16. The ends of thescrews are conically-shaped as is indicated at 2t) and 21, and theseconically-shaped portions mate with the conically-shaped portions 11 and12 of the insert l0 to respectively form the pivotsv p-l and p42. Withthis arrangement, the lever arm 6 can swing or tilt within the channelS.

As best seen in Figure 5, the lever arm 6 has a shoulder 22 whichcarries the knife edge k-l secured thereon by the screw 2d. The otherend of the lever arm 6 carries a screw 25, which is axially adju-stablein the lever and yadapted to be locked in adjusted position as by thenut 2 6. The end of the screw 2S is conically-shaped as is indicated bythe dotted lines 30.

Referring back t-o Figures l and 2, it will be noted that the bodyportion 2 carries .a bracket y3l secured thereon by the screws 32-32 andthe bracket mounts the housing 33 of the transformer T. The coils (notshown) of the transformer are enclosed within the housing together witha core element 34, the core being secured to a rod 35.

The upper end of the -rod 35 is conically-shaped, as indicated by thenumeral 36. This conical end mates with' the conically-shaped portion 30of the lever arm 6 t-o form a pivotal connection p-3 as between the rodand the 4lever arm. The lower end of the rod 36 is also conically-shapedas indicated at 40 and is supported by a spring 41, fixed by a screw 42,threaded in the body portion 2.

` Ordinarily the spring 41 has a small hole accommodating the tip of theconicalfly-shapedy portion 40 so as to provide for sure and positiveengagement.

TheV spring 4l tends to push the rod upward, hence, urge the lever in aclockwise direction. As best seen in Figure l, the motion of the levercauses the screw 24 to bear on the body l.

As seen in Figures l `and 5, the body portion 2 has a shoulder 43 inwhich is mounted the knife edge k-2, the edge being fixed to the body bythe screw 44. The tips of the two knife edges k-l and k-Z lie in thesame vertical plane as indicated by the dotted line P. The gauge lengthor distance between the knife edge tips may be set by appropriatelyfixing the distance which the screw 24 extends downwardly from the leverarm 6.

As best seen in Figure 4, the body portion 2 is provided with a slot 45,and in this slot is disposed a bracket 46 (see Figures 1 3), which isadjustably held in the slot by the washer 50 adapted to be tightened upby the screw 51. It will be apparent that by loosening or tightening 3the screw, the bracket 40 may be moved along the slot 45 and then be setin some desired position.

The bracket 46 `carries an anni 52 which is pivotally secured thereto bythe pivot 53. The arm carries the roller R rotatably mounted in the armand held in position by the screw 54.` The other end of the arm 52carries a lever 6l) which is fixed to the arm by the screw 61. A spring62 is secured to the lever by screw 63 and bears on the bracket 46. Theaction of the spring is to urge the roller arm against a specimen S sothat the specimen will be {irm against the knife edges k-l and k2.

As seen in Figure 6, the lever 60 can be moved toward the bracket 46 sothat the roller R moves away from the knife edge. This is ordinarilyaccomplished by placing the thumb on the bracket and the forenger on the|lever and squeezing. Thus, it will be apparent that the foregoing`arrangement provides a means for clarmping the extensometer on a testspecimen.

As mentioned above, the bracket 46 is adjustable, and it will beapparent that by adjusting the bracket, the roller may be made to assumea position closer or farther away from the plane of the knife edges.Thus, different size specimens may be easily accommodated. Itwill beobserved that the spring 62 is interposed between the lever and thebracket. Thus, for any adjusted position of the roller with respect tothe knife edges, the roller pressure developed by the spring remainsconstant.

I have found that the above-described pivotal interconnections p-l, p-2and p-3 enable the core to very accurately follow the motion of theknife edge k-l and to provide for linearity between the core and knifeedge. This is very important from the standpoint of accuracy,particularly where the over-all size of an extensometer is small. Insuch extensometers, the arm length, i. e., the distance between p-S andthe point of contact of the knife edge k1 and the specimen, m-ayonly bein the order of one inch, and it will be apparent that with such a smallabsolute length, the mechanical error in the linkage between the knifeedge and the c-ore must be kept to a minimum.

I claim:

1. An` extensometer adapted to be attached to a test specimencomprising: a mainframe having an elongated channel on the top thereof;a lever arm disposed in said channel and extending outwardly from eitherend thereof, one end of the lever having a knife edge and the other endbeing formed with a conical shape to provide one section of a pivot;interconnections between the lever and the main frame providing for thetilting of the lever in said channel; a second knife edge secured tosaid frame and extending outwardly `whereby the tips of the two knifeedges lie in the same plane; a transducer including coils fixed in saidframe and a core having an elongated rod, one end of the rod beingconically-shaped and engaging the conically-shaped portion of saidlever; a spring connected between the frame and the core and tending tourge said conically-shaped portions into engagement; a bracket securedto said frame and an arm pivotally connected with said bracket; a leverfixed on said arm and extending generally in the direction of saidbracket; a roller mounted on said arm and disposed generally inter:

4 jacent said knife edges; and a spring interconnected between saidlever and said bracket to normally urge the roller toward the plane ofsaid knife edges.

2. An extensometer adapted to be attached to a test specimen comprising:a main frame having an elongated channel on the top thereof; a lever armdisposed on said channel and extending outwardly from either endthereof; pivot means interconnecting the lever and the frame includingtwo conically-shaped portions in the lever and a pair of pins on saidframe respectively projecting into said channel, each pin having aconically-shaped head, the heads respectively mating with saidconically-shaped portions in the lever; a knife edge disposed on saidlever; a second knife edge secured to said frame, and arranged to lie ina plane containing said iirst knife edge, the knife edges being adaptedto be engaged by a test specimen; a roller disposed on said frame andadapted to engage a test specimen when the same is engaged with saidknife edges; a mechanism interconnecting the roller with said frameincluding spring means providing `for the roller to be normally urgedagainst a test specimen to push the same against said knife edges andfor the roller to be moved away from said knife edges.

3. An extensometer adapted to be attached to a test specimen comprising:a main frame; a lever arm disposed at the top of said frame andextending outwardly therefrom, one end of the lever having a knife edgeand the` other end having a screw, one end of which is conicallyshaped;a pair of screws mounted on the top part of said frame, one end of eachscrew being conically-shaped; an insert mounted in said lever arm whoseends are conically-shaped and respectively mount the ends of said pair0f screws whereby to provide a pivot for the tilting of the lever arm; asecond knife edge secured to said frame and extending` outwardly wherebythe tips of the two knife edges lie in the same plane; a transducerincluding coils fixed on said frame and a core having an elongated rod,one end of the rod being conically-shaped and engaging theconically-shaped portion of said lever arm screw whereby to provide apivotal connection as between the rod and the lever arm; a springoperatively interconnected between the frame and the rod and tending tourge the conically-shaped end of the rod and the conically-shaped end ofsaid lever screw into engagement; a bracket mounted on said frame andextending outwardly therefrom; an arm pivotally connected to saidbracket; a roller mounted on one end of said arm and disposed generallyinterjacent said knife edges; an operating lever fixed on the other endof said arm; and a spring operatively interconnected between said leverarm and said bracket providing for the roller to be urged against thetest specimen to push the same against said knife edges.

References Cited in the tile of this patent UNITED STATES PATENTS2,578,066 Hyde Dec. 11, 1951 FOREIGN PATENTS 289,203 Great Britain Apr.26, 1928

